From the early stages of providing voice-oriented services, mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems that provide data and multimedia services. Recently, various mobile communication standards such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), LTE-Advanced (LTE-A) of 3GPP, High Rate Packet Data (HRPD) of 3GPP2, and IEEE 802.16 have been developed to support high-speed, high-quality wireless packet data transmission services. In particular, an LTE system, which is a system developed to efficiently support high-speed wireless packet data transmission, maximizes wireless system capacity by using various radio access technologies. The LTE-A system, which is an advanced wireless system of the LTE system, has enhanced data transmission capability compared to the LTE system.
In general, LTE refers to a base station and user equipment corresponding to Release 8 or 9 of the 3GPP standard organization, and LTE-A refers to a base station and user equipment corresponding to Release 10 of the 3GPP standard organization. The 3GPP standard organization has standardized the LTE-A system and is now discussing the standard for the subsequent Release with improved capability based on the standardized LTE-A system.
The existing 3rd generation and 4th generation wireless packet data communication systems such as HSDPA, HSUPA, HRPD, and LTE/LTE-A employ an Adaptive Modulation and Coding (AMC) scheme, a channel-sensitive scheduling scheme, and the like to improve transmission efficiency. By using the AMC scheme, a transmitter can control the amount of transmitted data according to a channel state. That is, when the channel state is poor, the transmitter reduces the amount of transmitted data to adjust the reception error rate to a desired level. When the channel state is good, the transmitter increases the amount of transmitted data to adjust the reception error rate to the desired level and to efficiently transmit a large volume of information.
With the use of a channel-sensitive scheduling-based resource management method, the transmitter selectively provides a service to a user having a good channel state among a plurality of users, thus increasing the system capacity compared to the method of assigning a channel to one user and providing a service to the user with the assigned channel. This capacity increase is referred to as a multi-user diversity gain. That is, in the AMC method and the channel-sensitive scheduling method, partial channel state information is fed back from a receiver and a proper AMC scheme is applied at the time determined to be most efficient.
When the AMC method is used together with a Multiple Input Multiple Output (MIMO) transmission method, the AMC method may also include a function of determining the number or ranks of spatial layers of a transmitted signal. In this embodiment, in determining an optimal data rate, the AMC method also considers the number of layers to which data will be transmitted through MIMO as well as an encoding rate and a modulation method.
Research on switching Code Division Multiple Access (CDMA) corresponding to a multiple access method recently used in 2nd generation and 3rd generation mobile communication systems to Orthogonal Frequency Division Multiple Access (OFDMA) in a next generation system is being actively progressed. 3GPP and 3GPP2 starts progressing standardization for an evolved system using OFDMA. It is expected that the OFDMA scheme will have a larger capacity increase compared to the CDMA scheme. One of several causes which result in the capacity increase in the OFDMA scheme is that frequency domain scheduling can be performed. In the same manner as the channel-sensitive scheduling scheme in which a capacity gain is achieved according to the time-varying characteristic of a channel, it is possible to achieve a larger capacity gain by using the frequency-varying characteristic of a channel.
FIG. 1 illustrates time and frequency resources in an LTE/LTE-A system.
In FIG. 1, a radio resource which an evolved NodeB (eNB) transmits to a User Equipment (UE) is divided in the unit of Resource Blocks (RBs) on a frequency axis and in the unit of subframes on a time axis. In general, the RB consists of twelve subcarriers and occupies bands of 180 kHz in the LTE/LTE-A system. In contrast, one subframe generally consists of fourteen OFDM symbol intervals and occupies a time interval of 1 millisecond in the LTE/LTE-A system. The LTE/LTE-A system may allocate resources in the unit of subframes on the time axis and in the unit of RBs on the frequency axis in performing scheduling.
FIG. 2 illustrates radio resources of one subframe and one RB corresponding to a minimum unit which can be scheduled to the downlink in the LTE/LTE-A system.
The radio resource illustrated in FIG. 2 consists of one subframe on a time axis and one RB on a frequency axis. The radio resource consists of twelve subcarriers in a frequency domain and fourteen OFDM symbols in a time domain, and thus has a total of 168 inherent frequency and time locations. In the LTE/LTE-A system, each unique frequency-time location of FIG. 2 is referred to as a Resource Element (RE).
The following several different types of signals may be transmitted to the radio resources illustrated in FIG. 2.
A Cell specific Reference Signal (CRS) is a reference signal transmitted for all UEs included in one cell.
A DeModulation Reference Signal (DMRS) is a reference signal transmitted for a specific UE and is used for performing channel estimation to reconstruct information carried on a Physical Downlink Shared CHannel (PDSCH). One DMRS port applies the same precoding as that of a PDSCH layer connected to the DMRS port to perform transmission. A UE which desires to receive a specific layer of a downlink data channel (PDSCH) may receive a DMRS port connected to a corresponding layer to perform channel estimation and then reconstructs information carried on the corresponding layer using the channel estimation.
The PDSCH is used when the eNB transmits traffic to the UE through a data channel transmitted to the downlink and is transmitted using an RE to which the RS is not transmitted in a data region of FIG. 2.
A Channel Status Information Reference Signal (CSI-RS) is a reference signal transmitted for UEs included in one cell and is used for measuring a channel status. A plurality of CSI-RSs may be transmitted to one cell.
A Zero Power CSI-RS (ZP-CSI-RS) means that an actual signal is not transmitted to a position to which the CSI-RS is transmitted.
An Interference Measurement Resource (IMR) corresponds to a position to which the CSI-RS is transmitted. In FIG. 2, one or more of A, B, C, D, E, F, G, H, I, and J may be configured as the IMRs. The UE performs an interference measurement based on the assumption that all signals received in REs, which are configured as IMRs, are interference.
Other control channels (PHICH, PCFICH, and PDCCH) provide control information required when the UE receives the PDSCH or transmit ACK/NACK for operating HARQ with respect to data transmission of the uplink.
In addition to the above signals, the ZP-CSI-RS may be configured such that CSI-RSs transmitted by different eNBs can be received by UEs of the corresponding cell without any interference in the LTE-A system. The ZP-CSI-RS (muting) may be applied in a position where the CSI-RS can be transmitted and the UE generally hops the corresponding radio resource and receives a traffic signal. In the LTE-A system, the ZP-CSI-RS (muting) is also called muting. This is because the characteristics of ZP-CSI-RS demand that it be applied to a position of the CSI-RS and, therefore, transmission power is not transmitted.
In FIG. 2, the CSI-RS may be transmitted using some of positions indicated by “A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “I”, and “J” based on the number of antennas through which the CSI-RSs are transmitted. Further, the ZP-CSI-RS (muting) may be applied to some of the positions marked by “A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “I”, and “J”. Particularly, the CSI-RS may be transmitted to two, four, or eight REs according to the number of antenna ports transmitting the CSI-RSs. When the number of antenna ports is two, the CSI-RSs are transmitted to the half of a particular pattern. When the number of antenna parts is four, the CSI-RSs are transmitted to the entirety of the particular pattern. When the number of antenna ports is eight, the CSI-RSs are transmitted using two patterns. In contrast, the ZP-CIS-RS (muting) is always made in the unit of one pattern. That is, the ZP-CSI-RS (muting) may be applied to a plurality of patterns, but cannot be applied to only a part of one pattern when the ZP-CSI-RS does not overlap the CSI-RS. However, only when the CSI-RS and the ZP-CSI-RS (muting) overlap each other can the ZP-CSI-RS (muting) be applied to only part of one pattern.
“A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “I”, and “J” of FIG. 2 may be configured as IMRs. When the IMR is configured to a particular UE, the corresponding UE assumes signals received in Resource Elements (REs) included in the configured IMR as interference signals. The IMR is used for measuring strength of interference by the UE. That is, the UE determines strength of interference by measuring strength of signals received in REs included in the IMR configured for the UE itself.
Meanwhile, in a mobile communication system supporting FD-MIMO, the UE is required to measure interference from multiple users within the same eNB as well as interference from adjacent eNBs in order to improve the network reliability.